This invention relates to a method of color compensation exposure, and in particular, to a method of color exposure control for use in reproducing a positive photographic color picture from a positive color picture for printing purposes (referred to as color duplication hereinafter).
So-called color reproduction generally includes (1) electronic photographic reproduction, (2) reproduction of a positive image on color photographic paper from a negative photographic negative film (color printing), and (3) reproduction of a positive color image on a photosensitive duplicating film from a positive original picture (color duplication). Among such a variety of color reproduction, the last mentioned color duplication is the one which requires the highest level of color reproduction and is commonly used as an original picture for direct use in printing purposes.
In the field of color reproduction in general, the techniques for compensating color failure, which is a typical problem encountered in color printing, have been well known. However, in the field of color duplication, color failure of duplicating film rarely causes any problem but the problem of imperfection (bias in color tone) of the positive color original picture has been considered as a major problem which has to be compensated for somehow.
In most cases such color compensations are made with the use of color compensating filters. They are commercially available with indices ranging from 0 to 50 in multiples of 5 for each of the three basic colors cyan, magenta and yellow. Normally, two filters of two colors out of the three basic colors with suitable indices are used in combination for attaining a desired result from color compensation.
Therefore, when conducting a color compensating exposure for printing or duplication, it is necessary to select two filters out of a number of filters for effecting a desired color compensation after finding the colors for which a compensation is required and the required indices of the filters.
Such work is often very time consuming and skilled personnel is required for conducting such work. In particular, when producing so-called composite multi-color pictures in which a plurality of multi-color original pictures are arranged or laid out according to a plan on a single sheet of photosensitive film, the work involved becomes very time consuming since each original color picture generally has different color tones from the other pictures to be laid out together. No mistake is allowed in such work since one failure in color compensation in one original picture will ruin the whole composite color picture.
In the field of color the following methods for color compensation exposure have been either practiced or proposed.
The most basic method is the one in which a required kind of color compensation is first determined by visual inspection through a color viewer (using standard illuminating light) and, in addition to selecting the required colors of color compensating filters, their indices are selected.
Such a required kind of color compensation is usually expressed in terms of colors of filters and their indices such as 35C+25Y, 15M+20Y, etc. where the alphabetic terms indicate the color of filters, cyan C, magenta M, or yellow Y, and the numbers placed before them indicate their respective densities.
The selected color compensating filters are placed over the light source or at an appropriate location along the optical center line connecting the light source and a photosensitive material for an appropriate time period.
This method is very basic and simple in concept, but, since a great number of filters must be prepared in advance for effecting a wide range of color compensation, i.e. 10 filters for each of the three basic colors, at least, this method is rather complicated for an unskilled worker, and is not suitable for automatization either.
Another available method is the one using an integrating sphere. According to this method, each one of three color compensating filters having relatively high incides of the three basic colors, respectively, is independently thrust into the light path from a light source to an integrating sphere which is designed to accomplish a color mixture. A desired kind of color compensation is accomplished by selectively moving each of the three color compensating filters into the light path.
In this case, if the desired color compensation is of a kind which may be accomplished using only one color compensating filter, it may be easily done simply by adjusting the exposure interval of the particular color compensating filter according to the reading of a dial or the like. However, when more than one color compensating filter is required for accomplishing the desired color compensation, it is very difficult to obtain a desired quality of color compensation exposure since the two, color compensating filters will influence one another in a very complicated manner. Accordingly, some skill, which may be acquired only through much experience, will be required when putting this method into practice.
There is yet another possible method, which may be called a method of partial exposure times, according to which three, color compensating filters having relatively high indices of the three basic colors (50C, 50M and 50Y, for example), respectively, are prepared in advance, and the exposure is first made with only white light followed by additional exposures with one or two color compensating filters used in succession, either over the light source or at an appropriate location along the light path.
According to this method, the required kind of color compensation needs to be expressed in terms of allocation of total exposure time to each of the exposures (partial exposures), i.e. the exposure with only the white light and the additional exposures with one or two filters placed in the light path.
However, according to this method, a desired color compensation needs to be expressed in total exposure time and ratios between the allocated partial exposure times as opposed to the normal procedure in conventional color compensation exposures. There a desired color compensation is expressed in terms of colors and indices of color compensating filters which will produce the desired color compensating effect in such forms as 35C+25Y, 15M+20Y, etc. In the art of color compensation exposure, it has been the general practice to specify a desired color compensation in terms of a combination of indices of two color compensating filters which will produce a best result. Therefore, in order to make use of the method of partial exposure times, it becomes necessary to convert the indices of two color compensating filters into partial exposure times which will produce an identical color compensating effect.
Such a conversion has been generally believed to be impossible. In fact, in the method of partial exposure times, since the light from the light source will be attenuated by the color compensating filters due to their indices, which have certain spectral distributions, in a complicated manner, there has been no available method for determining the total exposure time nor the ratios between the partial exposure times. If, for example, the total exposure time is selected as the one which will produce a best result with the white light alone and it is allocated to the partial exposure times, not only does the overall exposure tend to be too little, but also it is very doubtful that a proper quality of color compensation will be attained. Therefore with this proposed method of partial exposure times, a skill which may be acquired only through much experience will become necessary.
However, according to this proposed method of partial exposure times, it is relatively easy to automatize a color compensation exposure, once the total exposure time and the ratios of the partial exposure times are known.
In consideration of the above described shortcomings of the conventional or proposed methods, one of the primary objects of this invention is to provide a method for color compensation exposure for color duplicators or the like which allows a desired kind of color compensation, which is defined in terms of a combination of filters with respect to their colors and indices, to be made according to an improved method of color compensation exposure based on a method of partial exposure times which is accurate in color compensation and easy to be automatized.
Such object is accomplished, according to this invention, by providing an improved method of partial exposure times comprising the steps of preparing three, color compensating filters having relatively high densities, respectively, selecting a primary color compensating filter out of the three, the color of which requires the greatest amount of color compensation, selecting a secondary color compensating filter, the color of which requires a secondary amount of color compensation, converting the densities of the two color compensating fiters with which the desired color compensation is defined into partial exposure times, comprising a first partial exposure time during which only white light is to be used for exposure, a second partial exposure time during which the primary color compensating filter is to be used over the white light, and a third partial exposure time during which the secondary color compensation filter is to be used over the white light in combination with the primary color compensating filter. An exposure made in three partial exposure times, each with a corresponding use of a color compensating filter or two, has the same effect as an exposure which is made with a use of two color compensating filters having two colors of appropriate indices, with which the desired color compensation is defined. The result is accomplished by an optimization of total effective exposure at a plurality of wave lengths selected from the visible range of light, and conducting three partial exposures in an arbitrary order, a first partial exposure only with the white light during the first partial exposure time, a second partial exposure with the primary color compensating filter placed over the white light during the second partial exposure time, and a third partial exposure with the secondary color compensating filter placed over the white light in combination with the primary color compensating filter.
This method is conveniently carried out with an apparatus according to this invention which comprises a film holder for holding a photosensitive material, a light source of a known spectral energy distribution, three color compensating filters having relatively high indices of three basic colors, respectively, arranged in such a manner that each of the filters may be selectively and individually displaced onto or away from the optical center line connecting the photosensitive material and the light source, a drive means for effecting the selective and individual displacement of the filters onto and away from the optical center line, a setting means for setting a required kind of color compensation in terms of indices and colors of two color compensating filters, a memory means for storing information concerning the optimum exposure time with the light source without a filter and properties of all the color compensating filters having the pertinent colors and indices, a computer means for determining a primary and a secondary color compensating filters out of the three color compensating filters having the relatively high indices of the three basic colors, the primary color compensating filter being required for a greater amount of color compensation than the secondary color compensating filter, as well as for computing partial exposure times, comprising a first partial exposure time during which exposure is to be made with the light source only, a second partial exposure time during which exposure is to be made with the primary color compensating filter placed over the light source, and a third partial exposure time during which exposure is to be made with the secondary color compensating filter placed over the light source in combination with the primary color compensating filter, in such manner that a color compensation which is equivalent in effect to the one which would be obtained with the combination of the two color compensating filters as set on the setting means, and a control means connected to the computer means for controlling, according to signals furnished from the computer means, functions of the drive means in such manner that the three partial exposures are conducted in a sequence of a predetermined order.
The above and other objects of the invention will become more clear from the following description of a preferred embodiment made by way of example in reference to the appended drawings.